Si(silicon)-based field effect transistors mainly used in semiconductor devices of today are of normally off type. A field effect transistor of normally off type is a transistor that is turned on in a case where a positive voltage is applied between a gate and a source, and turned off in a case where the positive voltage is not applied between the gate and the source.
Besides, GaN (gallium nitride) based and SiC (silicon carbide)-based field effect transistors, whose practical use is under study because of features such as high breakdown voltage, low loss, high-speed switching, high-temperature operation and the like, are of normally on type. A transistor of normally on type has a negative threshold voltage, is turned off in a case where a gate-source voltage is lower than the threshold voltage, and turned on in a case where the gate-source voltage is higher than the threshold voltage.
If such a field effect transistor of normally on type is used in a semiconductor device, various problems such as that a conventional gate drive circuit cannot be used and the like occur. Accordingly, it is proposed to compose a composite semiconductor device of normally off type by connecting a first field effect transistor of normally on type and a second field effect transistor of normally off type in series with each other.
For example, in a patent document 1, such a composite semiconductor device is disclosed, in which a drain-source voltage of the second field effect transistor of normally off type becomes higher than a breakdown voltage; and to prevent the second field effect transistor from falling into an avalanche state to be broken, a Zener diode is connected between the drain and the source of the second field effect transistor to limit the drain-source voltage to a voltage equal to or lower than the breakdown voltage.